1. Field of the Invention
The present invention relates to a bonding apparatus and more particularly to a bonding apparatus provided with an ultrasonic bonding horn used in manufacturing of, for example, semiconductor devices.
2. Prior Art
One type of conventional bonding apparatus, a nailhead heat-and-pressure bonding type wire bonding apparatus, is shown in FIGS. 5 and 6.
In this wire bonding apparatus, a supporting shaft 2 is fastened to a bonding arm 1 and is supported on a bonding head (not shown) either directly or via a lifter arm in a rotatable fashion. The horn support 4 of an ultrasonic horn 3 is mounted to the bonding arm 1. The ultrasonic horn 3 includes a horn body 6 which has a capillary 5 at one end and a vibrator 7 at another end. The vibrator 7 is screw-connected to the horn body 6. A bonding wire (not shown) passes through the capillary 5.
More specifically, the vibrator 7 includes a vibration-generating source 8 that is secured by screws. This screw installation is called a "Langevin" method. The vibrator 7 includes: a horn attachment 9 which is screw-connected to the horn body 6, a vibration-generating source attachment shaft 10 which has a threaded portion formed on both ends and is screw-connected to the horn attachment 9, an insulating pipe 11 which is fitted over the vibration-generating source attachment shaft 10, a vibration-generating source 8 which is obtained from a plurality of stacked doughnut-shaped electrostrictive strain elements or magnetostrictors that are fitted over the insulating pipe 11, and a nut 12 which is screw-connected to the vibration-generating source attachment shaft 10 so that the vibration-generating source 8 is tightened and secured between the nut 12 and the horn attachment 9.
In this conventional ultrasonic horn 3, the vibration-generating source 8 is located on the opposite side of the horn support 4 from the capillary 5. In other words, the vibration-generating source 8 is provided across from the capillary 5. The frequency of the vibrator 7 is adjusted to a desired level by the horn attachment 9 and the nut 12. The acoustic length of the vibrator 7 needs to be an integral multiple of 1/2 of the wavelength. Since there is no reason to use a lengthy vibrator, the vibrator 7 of the length equal to 1/2 of the wavelength is utilized. Furthermore, the free end 13 of the vibrator 7 acts as a vibrational antinode, and the horn attachment portion 14 of the horn attachment 9 acts as a vibrational antinode. Thus, the attachment of the vibrator 7 and the horn attachment 9 is facilitated.
In use, the vibration of the vibration-generating source 8 is transmitted throughout the entire ultrasonic horn 3 so that a standing-wave vibration is created in the ultrasonic horn 3, and the required energy is supplied to the capillary 5. In a non-loaded state (i.e., when bonding is not being performed), the energy accumulates in a stable manner. In addition, the horn 3 (particularly a skillfully crafted ultrasonic horn) is designed dimension-wise so that a node is formed in the horn support 4. Thus, the amount of movement of the horn support 4 is small and the loss of the movement is small even though the ultrasonic horn 3 is mounted to the bonding arm 1. In this non-loaded state, the ultrasonic horn 3 acts like a tuning fork, and the horn support 4 receives vibrations in a symmetrical manner from the left and right sides so that there is no movement to the left or right. The vibration-generating source 8 is ordinarily driven by constant-current driving, etc. so that the amplitude is kept at a prescribed value. When the energy is used for bonding via the capillary 5, the energy equilibrium between the vibration-generating source side and the capillary side loses its balance, and the node of vibration moves, and the energy required for equilibrium is fed in. An ultrasonic bonding is thus performed.
In the above-described ultrasonic horn 3, the horn support 4 is fastened to the bonding arm 1. Thus, the vibrational node rests on the horn support 4. Accordingly, the free end 13 and the horn attachment part 14 must be located a certain fixed length (such as 1/4 lambda, 3/4 lambda, 5/4 lambda, in which lambda is the length of sound waves) apart from the horn support 4 so that they can form a vibrational node. Furthermore, the shape and structure of the ultrasonic horn 3 are selected so that the horn support 4 does not interfere with the bonding work. The weight and strength of the ultrasonic horn 3 itself is also a critical matter in deciding the shape and structure of the ultrasonic horn 3. In the conventional ultrasonic horn 3, the vibration-generating source 8 is installed on the opposite side of the horn support 4 from the capillary 5. Also, a vibrational node is created in the center of the vibration-generating source 8, and the distance from the horn support 4 to the free end 13 is set to be 3/4 lambda in terms of the length of the sound waves. This structure has been accepted as a matter of common sense from the early date of wire bonding technology, and there has been no consideration given to the relationship of this length to the stability of bonding.
The factors which influence the vibrational characteristics are the magnitude of the energy that accumulates in an ultrasonic horn and the magnitude of the energy that is consumed. In the ultrasonic horn that uses electrostrictive strain elements or magnetostrictors as a vibration-generating source, sources of energy consumption in a non-loaded (or non-bonding) state include all mechanical loss around the periphery of the vibration-generating source because the vibration-generating source has numerous joined surfaces. However, when bonding (joining) via ultrasonic waves is performed, energy consumption at the surfaces between a workpiece and a bonding wire (both not shown) beneath the capillary and at the joined surfaces of the capillary and the horn body, etc. increases. This results in not only a progressive deterioration in Q value (an index which indicates how appropriate the vibration is) but also a deterioration in the characteristics of the vibrator. The reason for this is that the ultrasonic horn itself does not always have a single resonance point and the Q value of another frequency may become relatively high, so that the frequency jumps. The degree of deterioration in the Q value of a frequency, which is suitable for the vibration of the capillary, can change greatly during bonding and has no reproducibility. As a result, the energy is extremely difficult to control, and the vibration and amplitude of the capillary become either excessive or insufficient.
In the conventional ultrasonic horns 3, it has been extremely difficult to perform stable bonding if the temperature of the workpiece to be bonded is around 100.degree. C. which is low for a bonding temperature.
The reason for the difficulty in low temperature bonding is as follows: At low temperatures, the hardness of the metals to be bonded are high. In addition, since there is an increasing tendency to rely on ultrasonic waves both for deforming the ball formed at the tip of the wire and for obtaining a broad fresh surface by destroying any oxide film, any variation in the ultrasonic wave vibrational energy may lead to defective bonding if left as is.
Furthermore, depending on the conditions of the workpiece, there are some cases where bonding can be completed by the application of ultrasonic waves for a short period of time such as 5 ms, and other cases where bonding cannot be achieved unless ultrasonic waves are applied for a long period of time, such as 15 ms. As a result, it has been common to set the time on the longer side, so that bonding can be accomplished on all of the pads (electrodes of a workpiece to which the a bonding wire is to be bonded). However, once bonding has been completed, any subsequent vibration acts to destroy the bonded joint, and this may result in defects.
In conventional ultrasonic horns, the alteration of any part of the horn produces a difference in the results obtained, and various different approaches have been adopted to solve the problems. More specifically, in the absence of a proper understanding of the causes of bonding defects, it was believed that a more uniform ultrasonic horn would produce more uniform characteristics. Accordingly, there were repeated design attempts to place the vibrational node more precisely in the position of the horn support, and efforts were made to finish the joined surfaces of the constituent parts of the ultrasonic horn with greater precision. However, no improvement was obtained.
As a result of intense investigations of the causes of bonding defects in conventional bonding apparatuses, the inventors of the present application have found the following:
When an ultrasonic horn 3 is caused to vibrate in a gravity-free state, the free end 13 and the horn attachment part 14 should form a vibrational antinode and vibrate. In actual experiments, a state which is close to gravity-free is created by suspending the ultrasonic horn 3 from a string near the vibrational node so that no external forces are applied. In this case, the frequencies at which vibration is obtained correspond to resonance conditions in which the overall length of the ultrasonic horn is equal to 1/2 lambda, one (1) lambda, 3/2 lambda, etc. At the frequency determined in the design process, the vibrational node should be located in the horn support 4. In actuality, however, the vibrational node rarely coincides precisely with the horn support 4, and it is ordinarily impossible for such a positional coincidence to continue. In the past, there has been a tendency to believe that such a non-coincidence is caused by poor precision of the constituent parts of the ultrasonic horn 3. However, the inventors of the present application have ascertained that this is not an essential problem. PA1 Generally, the ultrasonic horn 3 is obtained from a horn body 6, which is made of a metal such as aluminum, stainless steel, etc., and a vibrator 7, which consists of electrostrictive strain elements, magnetostrictors, etc. Electrostrictive strain elements and magnetostrictors change their hardness according to mechanical stress and to the charge entering and leaving the electrodes of the vibrator 7. Accordingly, the resonance frequency also varies according to the assembly conditions and amplitude. In other words, since the frequency of such electrostrictive strain elements and magnetostrictors is not fixed at a certain level, the frequency of vibration varies, and the vibrational node shifts from the horn support 4. PA1 However, if the horn support 4 of the ultrasonic horn 3 is not fastened in place, i.e. placed in the gravity-free state (or state which is close to gravity-free), there is almost no change in the vibrational characteristics of the ultrasonic horn 3 even if the frequency of the vibration changes. PA1 On the other hand, if the horn support 4 of the ultrasonic horn 3 is secured in place, the horn support 4 is forcibly caused to become a vibrational node (where no vibration occurs). As a result, the front and rear parts (or the both ends) of the ultrasonic horn 3 have separate resonance frequencies. This difference in the resonance frequency alters the conditions of vibration of the capillary 5. PA1 Accordingly, the inventors of the present application reasoned that if the frequency of the constituent elements of the ultrasonic horn 3 varies depending upon the amplitude and temperature, it is necessary to develop a method of maintaining the correlation between the input current or voltage and the intensity of the vibration of the capillary 5.
Next, the causes of the problems will be explained further.
Even if the ultrasonic horn 3 has different resonance frequencies on the front and rear sides of the horn support 4, vibration will occur according to the frequency on the high-Q side if the Q value on the capillary side is lower than the Q value on the opposite side (i.e., the vibrator side).
However, when a bonding operation is performed, the energy consumption on the capillary side is increased, and the vibration on the capillary side of the horn support 4 assumes a type of "beat resonance" state, and the control becomes impossible. There are various reasons which lead to such a deleterious state as poor mechanical finishing precision, design errors, differences in the weight of the capillary, structural factors of the workpiece being bonded, and characteristics of the vibration-generating source 8 depending on temperature and amplitude, etc.
Despite the above circumstances, the inventors of the present application have ascertained by research that changes in the conditions of vibration caused by the application of a bonding load to the ultrasonic horn are not due only to poor precision of the constituent parts of the ultrasonic horn 3.